The fiber-reinforce composite material composed of a reinforcing fiber and a matrix resin is lightweight and has superior mechanical characteristics. For such a reason, the fiber-reinforced composite material is widely used as a structural material for aircrafts, automobiles, ships, construction and the like, as well as for sporting equipment such as golf shafts, fishing rods, tennis rackets and the like.
Various methods are used for producing the fiber-reinforced composite material. Among these, a method of impregnating a reinforcing fiber assembly with a matrix resin to thereby obtain a sheet like, a tape like, or a string like prepreg which is used as an intermediate base material is widely practiced. A thermally curable resin and a thermoplastic resin can be used as the matrix resin used for the prepreg, and the thermally curable resin is used more often.
A molded product (fiber-reinforced composite material) is obtained by laminating the prepreg in multiple layers, placing on a mold material, and then heating. Here, appropriate stickiness (tack property) of a surface of the prepreg can facilitate placing thereof onto the mold material and joining of the prepregs. In addition, if the position of the prepreg placed on the mold material is improper, correction of position is necessary. Therefore, excessive tack property is undesired on the surface of the prepreg.
If a molded product to be obtained has a curved shape, a curved mold material is used. Here, a rigid prepreg does not follow the shape of the mold material onto which the prepreg is placed. Therefore, appropriate flexibility is required for a prepreg.
For molding of a fiber-reinforced composite material using a prepreg, generally a plurality of prepregs of a given thickness is laminated to obtain the fiber-reinforced composite material of a desired thickness. Here, in a case of using the material as a structural material for vehicles such as ships, railway vehicles, and automobiles, as well as for windmills and the like, components are large in size and high in thickness. Given this, for such purposes, it is advantageous to use a prepreg high in thickness. A thick prepreg can be obtained by increasing the thickness of the reinforcing fiber assembly.
In production of the fiber-reinforced composite material, it is important to reduce voids generated due to portions, which are not impregnated with a matrix resin, inside of the material. If the prepreg has voids, the voids persist in the fiber-reinforced composite material, which is a molded product, and causes failure leading to reduced strength of the fiber-reinforced composite material. Therefore, it is necessary that the prepreg does not have voids.
Upon impregnation of the reinforcing fiber assembly with the matrix resin, exceedingly high viscosity of the matrix resin makes it difficult to impregnate the reinforcing fiber assembly with the matrix resin down to the inside thereof. In order to solve this problem, a so-called lacquer method and a varnish method have been proposed. In these methods, a thermally curable resin and a solvent are blended, the reinforcing fiber assembly is impregnated with the mixture, and then the solvent is removed by drying, thereby obtaining a prepreg.
However, in these methods, if the temperature of drying for removing the solvent is too high, the thermally curable resin cures at a stage of prepreg. In addition, this may dissolve or modify the curing agent in the thermally curable resin and a problem of shorter working life of the prepreg is likely. This problem is noticeable particularly in a case of using a thermally curable resin with a low curing temperature. In other words, in these methods, it is difficult to fully remove the solvent in the prepreg. If the solvent remains, the solvent is gasified during molding and causes voids in the fiber-reinforced composite material.
In order to solve this problem, a so-called hot melt film method has been proposed. In this method, a film of a thermally curable resin (resin film) is formed without using a solvent, the resin film is attached to a surface of, for example, a sheet-like reinforcing fiber assembly in which reinforcing fiber bundles are aligned, the sheet is heated to reduce viscosity and then pressurized to impregnate the reinforcing fiber assembly with the thermally curable resin, to thereby obtain a prepreg. However, in the hot melt film method, a thick reinforcing fiber assembly, more specifically a reinforcing fiber assembly having a weight of 300 g/m2 or greater, cannot be fully impregnated with the thermally curable resin from the surface to the inside.
In addition, high viscosity of the matrix resin may inhibit movement of the reinforcing fiber assembly during impregnation with the resin. Therefore, in the sheet-like reinforcing fiber assembly in which fiber bundles are aligned, if the reinforcing fiber bundles are not distributed evenly, surface smoothness of the prepreg after impregnation with the matrix resin is likely to be insufficient and colored patches are likely to be present in appearance. In order to avoid these problems, the impregnation must be performed under high pressure.
However, in a case of making the matrix resin enter into between monofilaments of the reinforcing fiber assembly under high pressure, so-called springback is likely to occur. The springback is a phenomenon in which the reinforcing fiber bundles, which have been compressed, gradually return to an original shape after the pressure upon producing the prepreg is removed, as if the fiber bundles remember the shape before the impregnation. Therefore, the hot melt film method is not appropriate for producing the thick prepreg.
Low viscosity of the matrix resin facilitates the impregnation of the reinforcing fiber assembly therewith. Given this, high temperature is not required during impregnation. In addition, since the reinforcing fiber bundles easily spread upon impregnation, high external pressure is not required. This can prevent the springback. In addition, since the impregnation with the resin can be preferably performed, the speed of a production line can be increased and productivity can be improved. Furthermore, the reinforcing fiber bundles with a high number of monofilaments can be used. In general, the reinforcing fiber bundles with a higher number of monofilaments are less expensive and advantageous in terms of cost. This also can reduce the number of reinforcing fiber bundles required, thereby improving productivity of the prepreg.
In a case in which the viscosity of the matrix resin is sufficiently low, methods other than the hot melt film method, for example, a method of impregnating the reinforcing fiber assembly with the matrix resin by a touch roller method, a dipping method, a dyeing method, a dispenser method and the like can be used. In these methods, it is not required to prepare a thermally curable resin film by a separate step. In addition, mold release paper, which is required in a case in which the viscosity of the matrix resin is high, is not necessarily required. These methods are cost effective since there is no need for: processing cost for an additional step; the mold release paper for supporting the thermally curable resin film; a protection film for protecting the thermally curable resin film; and the like. Furthermore, in these methods, an extremely thick prepreg, which is difficult to produce by a method using the mold release paper, can easily be produced.
However, in a case of using the matrix resin of low viscosity, the viscosity of the matrix resin in the prepreg to be obtained is also low, which has caused the following problems.
The tack property of the surface of the prepreg is too high and handling of the prepreg is difficult. In addition, the matrix resin may easily adhere to an operator and to a work area. Furthermore, the prepreg using the reinforcing fiber bundles aligned in one direction is low in proof strength against a force applied in a direction intersecting the direction of alignment of the reinforcing fiber bundles. This causes meandering of the reinforcing fiber bundles and breakage of the prepreg. In addition, it is difficult to correct an inappropriate position of the laminated prepregs.
In other words, there is a trade-off relationship between the viscosity of the matrix resin and the handling property of the prepreg.
As a resin composition that can solve the problem of trade-off, for example, Patent Document 1 discloses a resin composition comprising a thermally curable resin such as epoxy resin, radically polymerizable unsaturated compound, and a polymerization initiator that generates radicals in response to heating. In Patent Document 1, after impregnating the reinforcing fiber with the resin composition, the polymerization initiator is reacted therewith to generate radicals. Here, the resin composition with which the reinforcing fiber is impregnated is heat processed at a temperature lower than the temperature for curing the thermally curable resin, in order to increase viscosity of the resin composition in the prepreg.
However, in the method disclosed in Patent Document 1, a curing reaction of the thermally curable resin progresses in a part being heated. Therefore, the viscosity is increased not only at the surface but also in a central part of the prepreg. As a result, especially in a heavy weight prepreg, rigidity is remarkably increased due to increased viscosity of the matrix resin, leading to reduction in flexibility of the prepreg.
Reactivity of the polymerization initiator to heat processing after impregnation can be controlled by adding a polymerization inhibitor or a polymerization accelerator to the resin composition. However, the polymerization inhibitor or the polymerization accelerator makes the reaction complex. In addition, adding the polymerization accelerator may initiate a curing reaction of the thermally curable resin and may shorten a working life of the prepreg.